1. Technical Field
The invention relates to reinforced resilient pneumatic tires and more particularly to a vehicle tire reinforced by a thin high strength annular band which is stabilized by a plurality of radial elements in a tire sidewall to enable the tire to run in an unpressurized condition. More particularly, the invention relates to a method of building such a run flat tire wherein the band is coated with an elastomeric material and partially cured to enable the band to achieve a proper bond within the finished tire between the carcass and tread package.
2. Background Information
Various tire constructions have been devised over the years which enable a tire to run in an under-inflated or non-inflated condition, such as after receiving a puncture and loss of pressurized air, for extended periods of time and at relatively high speeds. This enables the vehicle operator to safely drive the vehicle to an appropriate location for repair or replacement of the punctured tire. Certain of these safety tires, referred to as "run flat tires", have been successful for certain applications and certain types of tire constructions. Most of these run flat tires achieve their run flat capability, by the placement of reinforcing layers or members of relatively stiff elastomeric material in the side walls of the tire which enable the tire to support the vehicle weight even with the complete loss of internal air pressure. Examples of such prior art run flat tire constructions which use such sidewall inserts are shown in U.S. Pat. Nos. 3,911,987; 3,949,798; 3,954,131; 4,067,372; 4,202,393; 4,203,481; 4,261,405; 4,265,288; 4,287,924; 4,365,659; 4,917,164; and 4,929,684.
Another type of run flat tire is referred to as a "banded tire." These banded tires have been promoted in literature and patents as a pneumatic tire reinforced by a radially stabilized compression element such that operation of the tire is independent of pressure. The compression element is commonly referred to as a band or band element, and as indicated above, tires incorporating this compression element are known as banded tires. Examples of such banded run flat tires are shown in U.S. Pat. Nos. 4,428,411; 4,673,014; 4,794,966; 4,456,048; 4,111,249; 4,318,434; 4,459,167; and 4,734,144.
Prior banded tires have concentrated primarily on the compression element consisting of at least one solid, thin annular band of high strength material, which behaves as a tension member when the tire is pressurized and which acts as a structural compression member when the tire is in the unpressurized state which allows loads to act over a substantial portion of the circumference of the tire. Various band designs achieve dual band stiffness capabilities suitable for the stress conditions imposed by both the pressurized and unpressurized tire states. Various methods have been developed to manufacture the band element. One of these methods imparts a prestressing of band fibers in order to improve band performance, described in pending application Ser. No. 08/782,364now U.S. Pat. No. 5,879,484. These band elements have various characteristics relating to dimensions, length, width and thicknesses and have preferred modulus of elasticity and resulting bending stiffness. A number of the above referenced patents disclose various methods for forming the band element.
However, one problem that exists is in the manufacturing of the run flat safety tires having the band incorporated therein. Although careful preparation may be utilized in preparation of the band, prior to its incorporation into the green tire, several problems occur when forming the finalized tire containing such a band. FIG. 1 shows a prior art tire containing a banded element when formed satisfactory with FIG. 2 showing a major problem which occurs during the formation of a run flat banded tire discussed further below.
Banded tires are made of either rigid or nearly rigid non-extensible circular bands which may be made of one or more of the known rigid non-extensible band elements formed of steel, aluminum, thermoplastic and thermosetting materials and multi-layered composites.
It has been determined that several difficulties must be overcome in order to successfully produce a banded tire on equipment intended for conventional radial tire productions. The two major problems that occur is the entrapment of air axially outwards towards the end of the band on the inside diameter as shown in FIG. 2 and the entrapment of air on the outside diameter of the band. Since the band element is rigid and changes imperceptibly from the green state to the final cure state, it results in the green tire profile of the banded tire being essentially the same as the final cure profile as shown in FIG. 1. The band is essentially flat across the crown portion of the banded tire in both the green tire and the cured tire and the sidewalls have minimum bulge in both cases. Thus, the green tire and cured tire profiles are approximately rectangular with axially extending crown portion and outwardly extending sidewalls. In other words, the shaping and expansion at the conventional second stage machine of a tire building process must deliver the final green tire profile as shown in FIG. 1.
In the manufacturing of a tire, the second stage tire building machine expands a first stage carcass outwardly and unites it with the band/tread package and then stitches the assembled pieces together, preferably without air entrapment. The banded tire first stage carcass consists of a usual innerliner, body plies, sidewalls and beads and when mounted on the second stage tire building machine will lie flat against the shaping bladder so as to be in a cylindrical or tubular configuration. Similarly, the band with the tread is positioned in an axial alignment over this assembly.
In operation, the tire forming bladder upon which the banded tire first stage carcass rests expands and moves the banded tire first stage carcass outwardly until it contacts the inside diameter of the band. Herein lies the first problem in that the body cords contained in the body ply are resistant to being expanded so as to transform from lying flat against the shaping bladder of the second stage tire building machine to the shape or expanded condition in order to maintain an essentially rectangular profile. This resistance of the body ply cords to assume a rectangular profile as shown in FIG. 1 is most noticeable near the axial ends of the band. The second stage shaping results in air being trapped between the inside diameter of the band and the adjacent first stage carcass near the band edges as shown in FIG. 2. This condition is unlikely to produce a usable tire in that curing the green tire with air trapped within the carcass is unacceptable.
Two other factors besides the natural resistance due to tension in the cords which are likely to contribute to the air entrapment between the inside diameter of the band and the adjacent rubber is as follows: The first relates to the difficulty in shaping the body cords so that they contact the band completely out to the edges of the band. This requires the second stage shaping bladder to be able to expand the body cords outward and into the required rectangular shape as shown in FIG. 1. However, the second stage shaping bladder relies upon pneumatic pressure for expansion and has limited ability to achieve a shaped rectangular profile. Even if reinforcement is utilized in the crown portion of the second stage shaping bladder, the rectangular profile is still difficult to achieve consistently. Thus, frequently this second stage shaping operation is unsuccessful, resulting in an imperfect banded green tire. This condition can be easily detected by non destructively examining the band edges on the inside of the tire.
Another contributing factor to air entrapment between the inside diameter of the band and the adjacent rubber within the banded green tire carcass relates to adhesion at the band/rubber interface. Even if the expansion as described above is successful, the body cords remain in tension because they have been expanded into a rectangular shape. This is especially true near the band edges, so that the body cords naturally want to pull away from the band. The only mechanism resisting this band/rubber separation is the green tack adhesion at the interface. However, the green tack adhesion between the band/rubber is weak in this uncured condition, since the cement designed to effect adhesion upon curing at the interface obviously hasn't been cured yet. Thus, even if the body cords are initially shaped into a rectangular profile as desired, the internal cord tension frequently overcomes the green tack adhesion at the band/rubber interface, thereby creating an air pocket.
In partial summary, the three prominent causes of air entrapment within the banded tire green carcass between the band inside diameter and adjacent rubber are effectively resolved by the present invention by enabling sufficient adhesion at this interface prior to the second stage shaping via pre-curing a layer of rubber to the band inside diameter.
Another problem with prior art banded tire manufacture is air entrapment/blistering on the outside diameter of the banded tire. This problem is more subtle in the green tire carcass and may not be apparent until after the tire emerges from the conventional curing press. Upon removal from the mold when the curing press opens, anomalies on the outside diameter of the banded tire will be apparent as blisters in the tread portion of the cured tire. This phenomena results when any pocket of trapped air at elevated temperature within the tire is allowed to expand, as when the tire is removed from the high pressure/temperature environment of the mold. Thus, any pocket of trapped air and/or region of poor adhesion at the band outside diameter/rubber interface will be magnified when the tire is removed from the mold and allowed to cool down. The most likely causes of blistering on the outside diameter of the banded tire are poor green tire adhesion at the band outside diameter/rubber interface; trapped air in the green tire carcass at the band outside diameter/rubber interface; and poor cured tire adhesion at the band outside diameter/rubber interface.
All of these causes are related to the rigid inextensible characteristic of the band, which is necessary for the functioning of the banded tire. All of these problems are successfully prevented by the method of the present invention which adheres a layer of rubber to the outside diameter of the band. Recall that the band outside diameter undergoes very little expansion from the green profile to the cured profile. Thus, any rubber movement associated with forming the tread pattern during curing necessarily is attributable only to mold segment movement. The mold segment movement necessary to transform the plain tread of the green tire carcass into the cured tire tread patters is very small. Therefore, the pressure environment on the outside diameter of the band created by closing the mold segments has a shorter time transient and a very sensitive pressure environment vs. conventional radial tires. The non-compliance of the band may create a very abrupt increase in viscous pressure as compared with the more compliant steel belts in a conventional radial tire, which allows trapped air to be relieved. Approaches to increasing the time transient during the tread forming process at curing and the high viscous pressures that may overstress the band are a mold closing pause of two seconds at partial tread pattern penetration; an additional mold cool down phase before mold opening; use of bleeder cords in the shoulder of the tire in order to provide a path of escape for trapped air; and puncturing the shoulders of the green tire prior to placement in the mold in order to provide a path of escape for trapped air. Thus, it is easily seen that the opportunity to consolidate or eliminate any entrapped air on the outside diameter of the band in the green tire is very limited during the conventional curing process.
There are two obvious ways air can get trapped on the outside diameter of the band during green tire construction. The first way is when the tread is stitched down to the band outside diameter at the second stage tire building machine. As explained above, if green tread rubber is stitched to the band outside diameter at the second stage tire building machine, the strength of this interface is limited to the green tack between the primed (adhesive cement) band and the uncured rubber. This interface can easily open, creating an entrapped air pocket unlikely to be consolidated during curing. However, it has been found that if a layer of rubber has been previously adhered to the band outside diameter, the stitching will be much more effective and the interface will not separate. This is because the green tack of green rubber to cured rubber is much greater vs. green rubber to primed band surface.
Another source of air entrapment on the outside diameter of the band relates to the previous discussion about air being trapped at the band inside diameter/rubber interface at the axial ends of the band. If this condition exists, the curing press bladder will apply high contact pressure to the inside of the green tire and literally drive any trapped air around the ends of the band to the outside diameter of the band. This is possible because of the low pressure environment on the outside diameter of the band during mold closure and curing, and the resultant large pressure differential between the band inside diameter and the band outside diameter. Any air thus driven to the band outside diameter likely will not be consolidated during curing, and will thus cause blistering.
Lastly, poor cured tire adhesion at the band outside diameter/rubber interface relates again to the low pressure environment at the band outside diameter/rubber interface during curing. In order to properly function, the cement present at this interface needs both sufficient temperature and pressure. If inadequate pressure is available during curing to fully develop adhesive strength at the band outside diameter/rubber interface, the interface can fill upon conventional mold opening. Again previously adhering a layer of rubber to the band, this problem can be averted. This is because the adhesive strength of the cured rubber/green rubber interface develops at a lower pressure vs. the primed band outside diameter/green rubber interface.
In summary, the three prominent causes of air entrapment/blistering on the outside diameter of the banded tire have been shown to be effectively resolved by the present invention by enabling sufficient adhesion to the band outside diameter/rubber interface prior to curing via pre-curing a layer of rubber to the band outside diameter.